Molecules and elements for quantitative bioanalysis: The allure of using electrospray, MALDI, and ICP mass spectrometry side-by-side

To understand biological processes, not only reliable identification, but quantification of constituents in biological processes play a pivotal role. This is especially true for the proteome: protein quantification must follow protein identification, since sometimes minute changes in abundance tell the real tale. To obtain quantitative data, many sophisticated strategies using electrospray and MALDI mass spectrometry (MS) have been developed in recent years. All of them have advantages and limitations. Several years ago, we started to work on strategies, which are principally capable to overcome some of these limits. The fundamental idea is to use elemental signals as a measure for quantities. We began by replacing the radioactive ³² P with the "cold" natural ³¹ P to quantify modified nucleotides and phosphorylated peptides and proteins and later used tagging strategies for quantification of proteins more generally. To do this, we introduced Inductively Coupled Plasma Mass Spectrometry (ICP-MS) into the bioanalytical workflows, allowing not only reliable and sensitive detection but also quantification based on isotope dilution absolute measurements using poly-isotopic elements. The detection capability of ICP-MS becomes particularly attractive with heavy metals. The covalently bound proteins tags developed in our group are based on the well-known DOTA chelate complex (1,4,7,10-tetraazacyclododecane-N,N',N″,N‴-tetraacetic acid) carrying ions of lanthanoides as metal core. In this review, I will outline the development of this mutual assistance between molecular and elemental mass spectrometry and discuss the scope and limitations particularly of peptide and protein quantification. The lanthanoide tags provide low detection limits, but offer multiplexing capabilities due to the number of very similar lanthanoides and their isotopes. With isotope dilution comes previously unknown accuracy. Separation techniques such as electrophoresis and HPLC were used and just slightly adapted workflows, already in use for quantification in bioanalysis. Imaging mass spectrometry (MSI) with MALDI and laser ablation ICP-MS complemented the range of application in recent years.

Synthesis and characterization of a new MeCAT reagent containing a photocleavable linker for labeling of proteins and peptides in mass spectrometric analyses

The quantification of proteins and peptides becomes more important besides mere identification in modern life sciences. Therefore, we have developed a new reagent that adds to the known metall-coded affinity tagging strategy employed in molecular and elemental mass spectrometry containing a photocleavable linker. A synthesis route was developed that provides the new reagent in good yields. The stability of the synthesized reagents was assessed under different temperature and illumination conditions. Labeling reactions were carried out at peptide and protein level, while also the fragmentation behavior of labeled peptides was assessed. In additional experiments, the photocleavability of the new reagent was examined. Upon irradiation with ultraviolet light, the photoproducts were liberated and could be used for quantification of labeled peptides.

Application of higher energy collisional dissociation (HCD) to the fragmentation of new DOTA-based labels and N-termini DOTA-labeled peptides

1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) derivatives are applied in quantitative proteomics owing to their ability to react with different functional groups, to harbor lanthanoides and hence their compatibility with molecular and elemental mass spectrometry. The new DOTA derivatives, namely Ln-MeCAT-Click and Ln-DOTA-Dimedone, allow efficient thiol labeling and targeting sulfenation as an important post-translational modification, respectively. Quantitative applications require the investigation of fragmentation behavior of these reagents. Therefore, the fragmentation behavior of Ln-MeCAT-Click and Ln-DOTA-Dimedone was studied using collision-induced dissociation (CID), infrared multiphoton dissociation (IRMPD) and higher-energy collision dissociation (HCD) using different energy levels, and the efficiency of reporter ion production was estimated. The efficiency of characteristic fragment formation was in the order IRMPD > HCD (normal energy level) > CID. On the other hand, the application of HCD at high energy levels (HCD@HE; NCE > 250%) resulted in a significant increase in reporter ion production (33-54%). This new strategy was successfully applied to generate label-specific reporter ions for DOTA amino labeling at the N-termini and in a quantitative fashion for the estimation of amino:thiol ratio in peptides. Copyright © 2017 John Wiley & Sons, Ltd.

Synthesis of d-labeled and unlabeled ethyl succinic anhydrides and application to quantitative analysis of peptides by isotope differential mass spectrometry

Ethyl succinic anhydride and its d₅-labeled version have been synthesized and applied to quantitative analysis of peptides in combination with MALDI or ESI mass spectrometry. These modifiers react with amino groups in the N-termini and lysine side chains in proteins, and therefore the combination of these modifiers was shown to be a useful tool for quantification of peptides and hence for proteomics research.

Detection of proteins by hyphenated techniques with endogenous metal tags and metal chemical labelling

The absolute and relative quantitation of proteins plays a fundamental role in modern proteomics, as it is the key to understand still unresolved biological questions in medical and pharmaceutical applications.

Quantification of intact covalently metal labeled proteins using ESI-MS/MS

Mass spectrometric methods matured from the successful qualitative characterization of proteins in complex mixtures into methods for quantitative proteomics often based on chemical tags with stable isotope labeling. In the study presented here, we extended the application of lanthanide-ion-based tags from the quantification using inductively coupled plasma-MS into the quantification of labeled intact proteins using electrospray ionization (ESI)-MS and ESI-MS/MS. We applied the metal chelate tag MeCAT-iodoacetamide (IA) (1,4,7,10-tetraazacyclododecane N,N',N″,N″ '-tetra acetic acid with a IA reactive site). Labeled proteins were separated using C3-reversed phase-high-performance liquid chromatography interfaced to ESI-MS. We could prove that even large proteins were completely labeled at all available cysteine residues using MeCAT-IA with only a small excess of reagent. Fragmentation of labeled proteins either using infrared multiphoton dissociation in Fourier transform ion cyclotron resonance-MS or higher-energy collision dissociation with an Orbitrap gave characteristic fragments. We used these fragments to quantify several intact proteins avoiding digestion. To demonstrate the applicability, human serum albumin was quantified in blood serum. The high-performance liquid chromatography/ESI-MS/MS quantification data were validated using inductively coupled plasma-MS. Because the metal within the tag may be any of the lanthanides, multiplexing capabilities are inherent.